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Ferroelectric size effect of BaTiO3 (BTO) tunnel junctions with metal Pt and/or oxide SrRuO3 (SRO) electrodes has been comprehensively investigated by the first-principle calculations. A vacuum layer is included in the supercell calculations, so that full-relaxation is achieved without artificial constraint on the supercell strains. We have constructed all of ten possible types of tunnel junctions with either symmetric or asymmetric geometries to systematically explore the influence of electrode/ferroelectric interfaces. The characteristics of atomic structure, polarization, charge density, and electrostatic potential for different geometries and sizes are revealed. It is found that the ferroelectric stability of a tunnel junction depends significantly on the details of the two electrode/ferroelectric interfaces, which present specific short- and long-range properties, e.g., local bonding environment, electronic screening, built-in field, etc. Result shows that Pt/BTO interfaces have strong coupling with ferroelectric distortion and thus play more dominant roles than the SRO/BTO interfaces in affecting the ferroelectric stability of the tunnel junctions. Particularly, it is found that Pt2/TiO2 interface can induce collective ferroelectric distortion in the initially non-distorted barrier. With a full-relaxation of the strains, an abnormal enhancement of ferroelectricity by Pt2/BaO interface due to Pt-O bonding effect is demonstrated, where a strong interfacial-bonding-related polarizing field is verified. Also importantly, polarization stability of asymmetric tunnel junctions is found dependent on direction, manifested with the appearing of a new critical thickness, below which the tunnel junction loses polarization bistability. Furthermore, it shows that the local features of a specific electrode/ferroelectric interface (e.g., the interfacial atomic structure, local polarization, charge transfer, and potential step) are well kept in different types of tunnel junctions. By analyzing and summarizing the results, our results suggest that traditional phenomenological models need several modifications in order to quantitatively reproduce the size effect of ferroelectric tunnel junctions. Our study provides a comprehensive picture of the ferroelectric size effect in BTO tunnel junctions as a function of electrode/ferroelectric interfaces and should have valuable implications for future studies and applications. [ABSTRACT FROM AUTHOR]

Based on the phase field simulations, effects of the surface charge screening and temperature on the vortex domain structure of the ferroelectric nanodot have been investigated. Our calculations show that the ferroelectric nanodot adopts a rhombohedral vortex domain pattern under an ideal open-circuit boundary condition. With the increase of the surface charge screening, the dipole vortex gradually rotates and appears rhombohedral-orthorhombic-tetragonal transformation. By adjusting the surface charge screening, the polar single domain and multi-vortices domain patterns with zero toroidal moment have been obtained near the phase transition temperature. More importantly, temperature and charge screening 'T-C' phase diagram has been summarized, which indicates an effective method to control the vortex domain structure in the low-dimensional ferroelectric nanostructures. [ABSTRACT FROM AUTHOR]

The stability of the remnant polarization in the ferroelectric barrier layer is a prerequisite to applications involving ferroelectric tunnel junctions (FTJs) or capacitors. One of the most important issues in the pursuit of further developments in this area is to overcome the limitations due to the critical thickness, below which the ferroelectric polarization disappears. In this paper we report first-principle density-functional calculations of the charge distribution and polarization in an asymmetric FTJ (A-FTJ), i.e., one with dissimilar electrodes. We found that a significant and stable polarization can be retained down to thicknesses as small as 0.8 nm (two unit-cells) in a BaTiO3 thin film between Pt and SrRuO3 electrodes, quite unlike the case of symmetric FTJs. We trace this surprising result to the large electric field produced by the charge transfer between the electrodes caused by their different electronic environments, which acts against the depolarization field and enhances the ferroelectricity, leading to the reduction, or even the complete elimination of the depolarization field, leading to the vanishing of the critical thickness. We speculate that this is a general result for A-FTJs, which could be of importance to applications of ferroelectric thin films and tunneling junctions or capacitors where the presence of the critical thickness is a limiting factor. [ABSTRACT FROM AUTHOR]

Photocurrent system of the ferroelectric nanocylinder (FNC), including nanodisks, nanorods, and nanowires, sandwiched between metal electrodes with the short-circuit boundary conditions has been designed and investigated. Taking into account the polarization charge screening in the electrodes and near-surface inhomogeneous polarization distribution, a theoretical model for investigating the photoinduced current of the FNC under the illumination of light was established. Our results show that the photocurrent of the FNC can be totally controlled by adjusting its size and states of the polarization “up” and “down.” Especially, reversing an applied stress can obviously change the photocurrent of the FNC, which is particularly significant near the stress-dependent para/ferroelectric phase transition. This piezophotovoltaic effect may have good potential for applications in high-sensitivity photomechanical sensors, memories, switchable nanodevices, or other photovoltaic nanodevices. [ABSTRACT FROM AUTHOR]

We present first-principles investigations on the structural, electronic, and polarization properties of Bi2ZnTiO6 using density-functional theory within the generalized gradient approximation. The theoretical structure we obtained confirms the extra large tetragonality observed by experiment. The materials exhibit a semiconductor behavior with an indirect band gap determined by the occupied O 2p and unoccupied Bi 6p states. There are strong hybridization effects between Bi–O ions, as well as Ti–O and Zn–O ones. The resulting covalent bondings, having a PbTiO3-type two-dimensional character, strengthen each other and favor the coupling between the tetragonal distortion of unit cell and the off-center displacement of A and B-site cations and O anions due to the existence of Zn, and result in the large tetragonality of this compound. Berry-phase calculation gives the polarization as high as 122 μC/cm2. [ABSTRACT FROM AUTHOR]

The critical behavior of a multilayered system of alternating ferroelectric and ferromagnetic thin films on a substrate of finite thickness is studied. A thermodynamic approach via the Ginzburg–Landau formulation is followed. Our interest is in the magnetoelectric effects exhibited in the critical behavior of the multilayer system, caused by the interlayer elastic interaction. The system is characterized by two bifurcation points. The system exhibits the strongest magnetoelectric effect near the second bifurcation. [ABSTRACT FROM AUTHOR]

We perform large-scale molecular dynamics simulations to study the magnetic properties of amorphous iron under pressure. Simulations, exceeding by at least two orders of magnitude those accessible to density functional calculations, use the recently developed magnetic interatomic potential for iron. The distributions of the size of atomic magnetic moments and parameters characterizing the structure of amorphous iron, such as radial distribution functions, are calculated as a function of the applied hydrostatic stress. As the density increases, there is a reduction in the magnitude of the mean magnetic moment of individual atoms, accompanied by the transformation of an increasing proportion of atoms from a magnetic to a nonmagnetic configuration. Beyond a critical density the proportion of nonmagnetic atoms increases sharply, yet homogeneously. The local magnetic moment of an atom correlates with the local Voronoi volume via a logarithmic relation. In addition, we observe a complex dependence of the local magnetic moment on the topological arrangement of neighboring atoms. [ABSTRACT FROM AUTHOR]

The stationary self-polarization field of a thin film in an open circuit is analytically solved for temperatures near the para-/ferroelectric transformation within the Ginzburg-Landau theory. For second-order ferroelectrics, or first-order ferroelectrics with a sufficiently large elastic self-energy of the transformation strain, the solution is real and stable, from which the corresponding electric susceptibility of the film can be derived. A Curie-Weiss-type relation of the permittivity is obtained for both the supercritical and subcritical temperature regimes near the transition. In the paraelectric state, the Curie parameter of the thin film is found to be independent of its thickness, whereas in the ferroelectric state, its magnitude decreases rapidly with decreasing film thickness. [ABSTRACT FROM AUTHOR]

The dynamic Ginzburg–Landau theory is applied to establish the critical conditions that control the transition between the paraelectric and ferroelectric states. Analytic expressions of the para-ferroelectric transition temperatures in a thin film under various electromechanical surface conditions are derived via a linear stability analysis of the evolutionary trajectory of the system for both first- and second-order transitions. Explicit expressions are then derived for the critical thickness, below which the thin film is paraelectric for all temperatures. For first-order transitions, the difference between the superheating and supercooling transition temperatures is found to be insensitive to the film thickness and surface boundary conditions. From these expressions, the relative importance on ferroelectricity in thin films due to applied mechanical constraints on the transformation strain and the depolarizing effect of surface charges is discussed and compared with experimental data. [ABSTRACT FROM AUTHOR]

The objective of this work was to understand the effect of the movable electrode (the tip of an atomic force microscope) on a piezoelectric-induced (PEI) image. Local polarization is induced on a lead zirconate titanate (PZT) thin film using an atomic force microscope (AFM), by applying dc voltage between the movable electrode and the Pt bottom electrode. The polarized PZT film is then characterized by the AFM using a two-pass method, in which both the topography and PEI image are obtained. The surface morphology is recorded in the first pass under contact mode with a fixed setpoint. A PEI image is obtained in the second pass in piezo-response mode. In this mode, the sample surface is scanned by applying ac voltage between the AFM tip and the Pt bottom electrode at sample displacement. PEI images of various sample displacement, corresponding to different stress exerted by the tip on the sample surface, are obtained and analyzed using force–sample displacement curves. It is found that PEI images can be detected if the tip adheres to the sample. The asymmetry of the A cos θ signal is improved as the force changes from repulsive to attractive. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this article, a general variational formulation is established for the dynamics of domain switching in ferroelectric thin films. By incorporating the kinetic energy in the Landau free energy expansion, we obtain a second-order time-dependent Ginzburg–Landau evolution equation, which can be used to analyze the frequency-dependent dielectric and ferroelectric properties of the materials. A numerical model is developed to calculate the time-dependent polarization and electric-field distribution during the domain switching process. As an example of practical importance, a ferroelectric film with 180° domain boundaries perpendicular to the film surface is studied. Domain switching is found to proceed in two stages, the initial nucleation and growth of the ferroelectric domain, followed by the expansion of the domain through the sidewise motion of the 180° domain walls, until the spontaneous polarization in the entire electrified area is reversed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ferroelectric random access memory (FRAM) has attracted much attention in the last two decades due to its ideal properties such as nonvolatility, high speed, and low power consumption. There is a strong incentive to develop high-density FRAMs, in which the switched domains, developed under low voltage or short pulses, are necessarily very small, and are therefore usually unstable and suffer from significant backswitching upon removal of the external voltage. In this investigation, a general form of energy expression for a ferroelectric material containing 180° domains is derived, from which evolution equations of the domain are established. By choosing the change in internal energy as the Liapunov function, a general formulation is developed to determine the stability conditions of the switched domain. This is applied to the case of an ellipsoidal 180° domain and yields a criterion for the stability of switched domains. We note that our approach is generally applicable to many other fields, including phase transformation, nucleation, expansion of dislocation loops in thin films, etc. [ABSTRACT FROM AUTHOR]

The polarization, charge offset, dielectric, and pyroelectric properties of a compositionally graded ferroelectric rod inside a high-pressure polyethylene tube are studied using a thermodynamic model based on the Landau-Ginzburg-Devonshire formulation. The calculated distribution of the polarization in the rod is nonuniform, and the corresponding charge offset, dielectric, and pyroelectric properties vary according to the applied pressure. This behavior may be used as a convenient means to control these properties for design optimization. [ABSTRACT FROM AUTHOR]